Patterns of accretion of reefs in the Sudanese Red Sea

Abstract

Reefs in the North Towartit and Shambaya Groups of the Sudanese Red Sea rest on major blocks which have been defined by large-scale fault movements. The dominant surface features in these areas have been formed by the sub-aerial sculpting of Tertiary and later limestones during late Pleistocene periods of low sea-level stand. This has resulted in the formation of large numbers of closed depressions reaching −30 m depth. These punctuate a surface lying between −20 and −30 m which is itself probably a residual of an older, undulating, higher level surface. The margins of depressions are submarine cliffs, the smooth floors are blanketted with Recent sediments while the intervening ridges have become the sites for reef accretion. Present coral growth began about 6000 years ago and has reached a thickness of 10 to 20 m. It has been limited in deeper water but in shallower areas may have been aided by establishment on existing pinnacles. Steep margins occur on both the windward (Baraja) and leeward (Towartit, Harvey) margins of reefs. These may result from inequalities of growth but cannot be directly related to environmental energy. Windward and leeward shores are characterised by contrasted coral associations, a windward Acropora association and a leeward Porites association. This distribution reflects a depletion of resources downcurrent rather than major differences in water movement. Reef edges are the most important sediment-generating zones and patterns of dispersal point to a dominant westward (downwind) movement. 50–80% of sediments coarser than 1 ø are coral and algal fragments. Mollusc shells are a minor component but reach 26% on the Towartit fringe. Homotrema, benthic forams, bryozoa, crustacean and echinoderm fragments rarely rise above a few percent each. Mean sizes of sediments range from +2.3167 to −1.2333 ø with the coarsest close to the reef edges and the finest in deep water or down current. Most are poorly sorted and values for standard deviation range between +2.1714 and +0.9512 ø. Skewness values indicate symmetrical or coarse skewed distributions (−0.0132 to −0.4982). Finally, figures for normalised kurtosis range between 0.6023 and 0.4532. These suggest that sediments consist of at least two sub-populations: (1) a base stock generated in the edge zone with characteristics varying systematically down current; and (2) grains formed within the transport system with characters owing more to ecological factors than hydrodynamics. The irregular mixing of these two accounts for the variation shown. In conclusion, the character of the reefs depends both upon present-day processes and an inherited morphology.